(1) Field of the Invention
The present invention relates to a frame transmission device, and more particularly, to a frame transmission device for causing supervisory frames to enter subscriber traffic to be transmitted along with subscriber frames.
(2) Description of the Related Art
With the recent advance of multimedia, the demand for information communication networks is becoming higher in level and diversifying, and thus the development of network communication technologies is being swiftly forwarded to realize advanced information-oriented society. The mainstream of currently configured networks is IP (Internet Protocol)-based network typified by the Internet, and further improvement in serviceability of IP networks is demanded.
IP network has evolved as a best effort type network (form of communication in which best efforts are exerted to minimize delay, packet loss, etc.), but because of explosive spread of the Internet, what users demand from carriers (telecommunication carriers) is shifting from the conventional “best effort” to “QoS (Quality of Service)”.
For example, in the case of VoIP (Voice over IP) service whereby voice is transmitted over an IP network, even a best effort-based IP network is required to provide QoS that can maintain a certain level of speech quality such as ease of hearing and ease of conversation.
To provide such high-quality communication service, it is necessary to monitor the QoS of the network so that when the quality degrades due to network congestion or the occurrence of fault, for example, such degradation can be promptly detected to restore the quality to a normal level. Thus, network supervision such as QoS monitoring is acquiring a greater importance and the development of network supervisory systems with high monitoring functionality is strongly demanded.
FIG. 16 illustrates the configuration of a conventional network supervisory system. An IP network 200 as an upstream-side network and an Ethernet (registered trademark) 230 as a downstream-side network are connected by repeaters 210 and 240. The Ethernet 230 is provided with hubs Ha and Hb.
First, the connection of a transmission line (link) L1 for primary signal (subscriber frames) will be explained. The repeater 210, which is provided at the edge of the IP network 200, connects with the IP network 200 through a LAN port Lp1a. The hub Ha connects with the repeater 210 through a port ha1.
The hubs Ha and Hb are connected to each other through a subscriber traffic line in the Ethernet 230. The hub Hb has a port hb1 connected to a LAN port Lp2a of the repeater 240 which is connected to a subscriber terminal 250. Using the transmission line L1 connected in this manner, the primary signal is exchanged between the subscriber terminal 250 and the IP network 200.
Supervisory lines L2a and L2b for monitoring the repeaters 210 and 240 are connected in the manner described below. When monitoring the repeater 210, an SNMP (Simple Network Management Protocol) manager 220, which is a supervisory terminal, connects with an RS232C port of the repeater 210 to acquire supervisory information in the repeater through the supervisory line L2a. 
When monitoring the repeater 240 which is located at a remote subscriber's site, the SNMP manager 220 connects with a port ha2 of the hub Ha. Then, the hubs Ha and Hb connect with each other through a supervisory control line in the Ethernet 230, a port hb2 of the hub Hb connects with a LAN port (in FIG. 11, Line port side) Lp2b of the repeater 240, and the repeater 240 connects with the subscriber terminal 250. Using the supervisory line L2b connected in this manner, the SNMP manager 220 acquires supervisory information in the repeater 240 located at a remote place.
There has also been proposed a network supervision technique in which, when monitoring an optical wavelength division multiplexing network, one wavelength is allocated to transmission of supervisory control signal to monitor network faults (e.g., Japanese Unexamined Patent Publication No. 2003-298540 (paragraph nos. [0022] to [0070]; FIG. 1)).
In the conventional network supervisory system explained with reference to FIG. 16, when the status of the remote, downstream-side repeater 240 is to be monitored through the upstream-side repeater 210, the supervisory control line L2b needs to be provided separately from the primary signal line L1. Accordingly, the conventional system requires the adjustment of a network interface environment to allow the connection of the supervisory line L2b, giving rise to a problem that the monitoring function cannot be easily introduced. Also, the separately provided supervisory line occupies LAN ports and hub ports for its own purpose, so that the number of idle ports decreases, causing disadvantages such as insufficiency of communication port resources for the primary signal.
In network supervisory control for TDM (Time Division Multiplex) systems such as ATM (Asynchronous Transfer Mode) systems and SDH (Synchronous Digital Hierarchy) systems, supervisory information is caused to enter the primary signal line, instead of separately providing a supervisory line, to perform supervisory control. Similar supervisory control is also performed in the aforementioned conventional technique (Japanese Unexamined Patent Publication No. 2003-298540) by allocating one wavelength to supervision.
However, in the conventional supervisory control wherein supervisory information is caused to enter the primary signal line, fixed timing or fixed slot or wavelength is allocated to supervision, with the result that part of the bandwidth originally allocated to users is always used as a supervisory bandwidth. If a fixed bandwidth is always secured for the supervisory information as in this supervisory control, the users' physical rate lowers by an amount corresponding to the supervisory bandwidth, posing a problem that the quality of service for the users lowers.